Acquisition of GPS C/A Code Signals - Fundamentals of Global Positioning System Receivers: A Software Approach - page 130

strong, the fast, low-sensitivity acquisition method can find it. If the signal is weak,
the low-sensitivity acquisition will miss it but the conventional method will find
it. If the signal is very weak, the long data length acquisition should be used. A
proper combination of these approaches should achieve fast acquisition. However,
a discussion on combining these methods is not included in this topic.
Once the signals are found, two important parameters must be measured. One
is the beginning of the C/A code period and the other one is the carrier frequency
of the input signal. A set of collected data usually contains signals of several
satellites. Each signal has a different C/A code with a different starting time
and different Doppler frequency. The acquisition method is to find the beginning
of the C/A code and use this information to despread the spectrum. Once the
spectrum is despread, the output becomes a continuous wave (cw) signal and its
carrier frequency can be found. The beginning of the C/A code and the carrier
frequency are the parameters passed to the tracking program.
In this and the following chapters, the data used are collected from the down-
converted system. The intermediate frequency (IF) is at 21.25 MHz and sampling
frequency is 5 MHz. Therefore, the center of the signal is at 1.25 MHz. The data
are collected through a single-channel system by one analog-to-digital converter
(ADC). Thus, the data are considered real in contrast to complex data. The
hardware arrangement is discussed in the previous chapter.
7.2 ACQUISITION METHODOLOGY
One common way to start an acquisition program is to search for satellites that are
visible to the receiver. If the rough location (say Dayton, Ohio, U.S.A.) and the
approximate time of day are known, information is available on which satellites
are available, such as on some Internet locations, or can be computed from a
recently recorded almanac broadcast. If one uses this method for acquisition, only
a few satellites (a maximum of 11 satellites if the user is on the earth's surface)
need to be searched. In Section 12.2 it is illustrated that coarse information for
acquisition can be obtained from almanac data. However, in case the wrong
location or time is provided, the time to locate the satellites increases as the
acquisition process may initially search for the wrong satellites.
The other method to search for the satellites is to perform acquisition on all
the satellites in space; there are 24 of them. This method assumes that one knows
which satellites are in space. If one does not even know which satellites are in
space and there could be 32 possible satellites, the acquisition must be performed
on all the satellites. This approach could be time consuming; a fast acquisition
process is always preferred.
The conventional approach to perform signal acquisition is through hardware
in the time domain. The acquisition is performed on the input data in a continuous
manner. Once the signal is found, the information will immediately pass to the
tracking hardware. In some receivers the acquisition can be performed on many
satellites in parallel.
When a software receiver is used, the acquisition is usually performed on a
block of data. When the desired signal is found, the information is passed on to